test(asyncapi): use a simple rule tester

[P0lip]

Related to #1082 and #1618

Checklist

• Tests added / updated
• Docs added / updated

Does this PR introduce a breaking change?

• Yes
• No

Additional context

This essentially removes all the boilerplate and burden we needed to test a particular rule.
Right now, that logic is hidden in a single file, while each test is actually oriented on the expected output.

I'm planning to apply the same change to the OAS ruleset tests.

Later on, when built-in rulesets have new home, we'll either leave a util in Spectral (I bet this will be super useful for other rulesets too, in a similar fashion to the ESLint offers), or move the util altogether to the new repo.

I feel like it doesn't make sense to review each file - verifying the approach is what we should put more focus on.
Thea idea is that instead of initializing Spectral with a ruleset, and then writing assertions manually, etc., you simply rely on testRule to do all of that.
This way we're totally independent of changes occurring to Spectral.

import { DiagnosticSeverity } from '@stoplight/types';
import testRule from './__helpers__/tester';

testRule('asyncapi-channel-no-empty-parameter', [
  {
    name: 'valid case',
    document: {
      asyncapi: '2.0.0',
      channels: {
        'users/{userId}/signedUp': {},
      },
    },
    errors: [],
  },

  {
    name: 'channels.{channel} contains empty parameter substitution pattern',
    document: {
      asyncapi: '2.0.0',
      channels: {
        'users/{userId}/signedUp': {},
        'users/{}/signedOut': {},
      },
    },
    errors: [
      {
        message: 'Channel path should not have empty parameter substitution pattern.',
        path: ['channels', 'users/{}/signedOut'],
        severity: DiagnosticSeverity.Warning,
      },
    ],
  },
]);

[domagojk]

Much nicer 👌